The present invention relates generally to electrical test instruments, and more particularly to a method and apparatus for testing capacitors (particularly electrolytic capacitors) by measuring their ESR (Equivalent Series Resistance) in-circuit, i.e., without removing the test capacitor from its circuit and without first discharging the capacitor prior to testing.
Recent research experience on field failures of electrolytic capacitors has proven that almost all of them (approximately 99 percent) failed because of ESR (Equivalent Series Resistance). The internal resistance reduces the capacitor's rate of charging and discharging, effectively making it an open circuit. High ESR usually results from dehydration of the electrolyte in the capacitor due to equipment heat, old age, poor sealing, or internal heat generated from ESR and high ripple currents.
Another common reason for high ESR is defective terminations due to broken welds, loose crimps, or rivets and/or corrosion. These problems cause variable ESR or intermittent opens and can usually be detected by monitoring ESR while wiggling the test capacitor. This inventor has yet to find an electrolytic capacitor with normal ESR readings whose circuit failure was due to a change in capacitance alone. This is why the capacitance or capacity meters of the prior art have failed to solve the problem and are seldom, if ever, used outside of schools, engineering laboratories or the like. The real problem, unrecognized by the service industry in general for over 50 years, is not the change in the capacitance of an electrolytic capacitor, but rather ESR changes!
Only about one percent of all of today's electrolytic capacitor failures are due to leaky or shorted capacitors and both of these types of failures are easily spotted since they cause serious circuit voltage changes and often result in burned parts, or the like. The other 99 percent of all failures can only be detected in the field with some type of ESR meter or by substitution.
Conventional test devices include capacitance or capacity meters which are of little or no value, especially in the field, where less than one percent of all failures can be detected with them, as previously described. These meters are quite complex and expensive and find little or no use in the real world of installation, maintenance or repair of electrical circuits in the field.
AC volt meters can be used to measure circuit performance and detect problems but will not permit the service man to determine the exact location of the problem nor tell him, in all cases, whether or not a particular electrolytic capacitor has failed or is about to. DC ohmmeters can't be used at all to measure the resistance of capacitors since a capacitor does not pass DC current.
Recently a large corporation and major supplier for the electronics service industry came out with a capacitor analyzer which (1) measured the capacity of a capacitor after it had been removed from its circuit and discharged and (2) evaluated the general performance of electrolytic capacitors by placing the out-of-circuit, discharged, electrolytic capacitor to be tested in a simulated test circuit and evaluating it based on (a) its measured capacitance, (b) its leakage current; and (c) its out-of-the-circuit, fully discharged Equivalent Series Resistance. However, this analyzer did not succeed in the field due to (1) the need for first discharging the capacitor; (2) the need for physically removing the test capacitor from the circuit in which it is used for testing; and (3) its extreme electrical complexity and cost. Therefore, it found no use in the service industry, and is no longer even offered for sale in the market place.
A more recent attempt has resulted in a complex digital tester for laboratory use in measuring many different capacitor conditions including the out-of-circuit ESR of a capacitor, but the digital tester is extremely large, complex and costly and therefore will never find general acceptance for use in the field.
Capacitor testing today is usually done in the field (1) by paralleling a suspected bad capacitor with a known good one or (2) by substituting a good one in place of the suspected capacitor and seeing if the basic circuit performs as required. Such methods are very time consuming, costly and unreliable and often lead to blown out semiconductor components elsewhere in the circuit, as known in the art.
One of the most basic of all of the problems of the prior art lay in the fact that the prior art completely failed to recognize the usefulness of ESR in testing capacitors. For example, the prior art taught away from the use of ESR as an efficient method of testing electrolytic capacitors because the prior art specifically emphasized tht ESR is not a "pure" resistance in that it is somewhat affected frequency and is related to capacitance and hence design, and therefore, no one thought it could be measured accurately enough to be useful on a service basis for anything, let alone as a basis for testing capacitors. Besides, the fact that the failure of electrolytic capacitors was not recognized as being chiefly due to dehydration of the enclosed electrolyte or other failures causing a high ESR, was not recognized nor exploited prior to Applicant's invention.
For these reasons, while electrolytic capacitors have been used extensively for over 50 years, no one has heretofore developed a practical and reliable method and apparatus for testing capacitors in the field during installation, maintenance or other service-type operations. Applicant has filled this long-felt need by going against the teachings of the prior art, and solving substantially all of the problems of the prior art with the present method and apparatus for testing electrolytic capacitors in-circuit by measuring their ESR as an indicia of the reliability of the capacitor without first discharging the test capacitor and without removing the test capacitor from the circuit in which it is being used prior to testing.